German patent document No. 3,048,941 filed Dec. 24, 1980 by F. Steck describes a mortar grenade having, as is standard, a hollow casing having a nose and a tail and filled with a charge that explodes when a concussion primer in the nose is detonated, that is when the grenade lands with great force on its nose end. The tail carries the propulsion charge and is provided with four stabilizer vanes that can pivot about respective axes angularly spaced about the longitudinal axis of the casing. Each of these vanes can pivot between a position lying flatly against the tail and in fact within the longitudinal axial extension of the main front part of the casing and a position projecting radially of the casing axis beyond the casing. Springs urge the vanes into the outer position and a cardboard retainer sleeve secures the vanes normally against this spring force in the inner positions. The vanes are in the inner position until the projectile leaves the muzzle of the barrel or tube from which it is shot, whereupon these vanes erect or deploy to the outer position in which they serve to stabilize the flight of the projectile through the air. On firing, the propulsion charge burns off the cardboard retainer, allowing the vanes to pivot out and thereby serve to stabilize the projectile in flight.
Such a system has the disadvantage that the space taken up by the sleeve cannot be filled with charge to maximize the explosive force of the projectile. This problem increases with high-speed projectiles with a muzzle velocity greater than 800 m/sec where the vanes are pivotal about axes lying in a plane perpendicular to the casing axis and all extending tangentially of a circle centered in this plane on the casing axis. Each vane in this high-speed system can rotate about the respective axis between an inner position pointing forward and recessed in a respective slot in the casing and an outer position projecting radially from the casing. The sleeve for such a high-velocity projectile must be fairly tough to withstand the considerable inertial forces it is subjected to, so that the propellant charge is generally reduced for such a grenade to thereby decrease its muzzle velocity to less than 600 m/sec, thereby gentling the sleeve and giving it time to burn off. The thicker sleeve in this system has the further disadvantage that it takes up even more valuable space, reducing the amount of charge the grenade can carry.